1. Field of the Invention
The invention relates to DC-to-DC converters.
2. Description of Prior Art
In DC-to-DC conversion with isolation between input and output, conventional forward and flyback converters are commonly used in the industry. Single-ended flyback converters are dominant in the low-cost and low-power power supply applications because of their circuit simplicity. Furthermore, they are frequently used for wide-input-range power converters, for example an AC adaptor with universal input voltage range (90-240 VAC), because of their nonlinear output-input voltage conversion ratios (D/(1-D)). Synchronous rectifiers are used to improve power conversion efficiency, especially for low output voltage, such as described in U.S. Pat. No. 4,958,268 in a flyback converter.
U.S. Pat. No. 4,958,268 also discloses a special type of resonant-transition switching using the synchronous rectifier (FIG. 13). A reverse magnetizing current is produced by the reverse conduction of the synchronous rectifier before the primary switching means turns on. The parasitic capacitance across the primary switching means is discharged by the reverse magnetizing current, and thus the turn-on switching loss can be minimized. Small magnetizing inductance or large flux excursion of the flyback transformer is required to achieve this resonant-transition switching; higher core loss usually results. High RMS currents in both primary and secondary windings also cause high conduction loss in the flyback transformer. Moreover, a high ripple current flowing through the output capacitor causes high ripple voltage and spikes. An L-C output filter is usually required for such a special type of flyback converter.